Metal containers for receiving foods and beverages generally have one or more coatings to prevent contact between the filled product and metal. This is to prevent or minimize corrosion to the metal by the product and any disadvantageous influences on the quality of the product. For producing containers of this type, such as cans of tin-plated or chromium-plated steels or aluminum, metal sheets are used which, prior to their shaping (such as for three-piece can production) or deformation (such as for deep drawing process), are coated with suitable coating compositions. In producing cans for foods and beverages, coatings are required which are extremely flexible and have a low order of toxicity. Furthermore, filled food cans are often sterilized under application of temperatures up to 135° C. The coating must therefore be sufficiently stable at those temperatures an must be capable of adhering to the surface of the metal.
Epoxy phenolic type coatings have been applied as lacquers onto metal can stock (e.g. for three-piece cans) and baked to provide coatings having good resistance to aggressive filled products, mechanical performance and metal adhesion. However, many of these incorporate 2,2′-bis(4-hydroxyphenyl)propane-bis(2,3-epoxypropyl)-ether (or homologues thereof), otherwise known as bisphenol-A-diglycidyl-ether or “BADGE” (Bisphenol-A-Di-Glycidyl-Ether).
The use of BADGE-containing formulations is restricted under food law aspects.
Currently available polyester type coatings, such as those cross-linked with amino-type or isocyanate-type resins, are used on the exterior of three-piece cans, but do not resist processing when in contact with food, or do not comply with food laws, and therefore do not yet provide an alternative coating formulation.
WO 00/55265 describes BADGE-free compositions, which are, however, produced on the basis of polyester resins in combination with bisphenol-A (BPA). Organosoles on the basis of PVC crosslinked with bisphenol-A serve as an alternative to polyester resins. Bisphenol-A was therefore preferably used for the production of sterilization resistant, epoxy-free can coatings. However, it has been found that bisphenol-A acts to the human body in a similar manner as estrogen, which may lead to reproduction disorders.
Crosslinked coating compositions of the polyvinylchloride type are commercially available. However, all known sterilization-resistant coating compositions contain BADGE and/or BPA. The term “cross-links”, as used within the context of the present invention, designates covalent bonds linking one polymer chain to another and the term “crosslinking agents” designates monomers or polymers that are able to react or co-react with the polymer chains linking them together to form a duroplastic network.
Many known coating compositions for metal cans also contain metal stabilizers such as lead- or tin-based stabilizers. These metal stabilizers are undesirable in cans for foods and beverages and do not comply with food laws.
Furthermore, in many known coating compositions it is necessary to first apply a primer composition onto the metal surface, before applying the coating composition. This is disadvantageous because a further, costly processing step is required, and because many primer compositions do not comply with food laws.
In view of the foregoing disadvantages of the prior art, a novel can coating is need which is substantially BADGE- and BPA-free.